Multi-layer paints and method for producing the same

ABSTRACT

The present invention relates to a multilayer coating comprising at least one basecoat layer and one topcoat layer, the coating being preparable by 
     A. applying a component (I), if desired radiation-curable component, based on polyurethane resins, polyacrylate resins, polyester resins and/or amino resins and also mixtures thereof to a substrate and subjecting it to thermal partial crosslinking or drying at temperatures of less than 100° C., preferably 60-80° C., and 
     B. applying a topcoat to this coating layer, and subsequently carrying out radiation curing.

Subject-matter of the present invention are multilayer coating systems,their production, and the use.

PRIOR ART

In past years, great progress has been made in the development of acid-and etch-resistant clearcoats for automotive production-line (OEM)finishing. More recently there is now an increasing desire in theautomotive industry for scratch-resistant clearcoats which at the sametime retain the level achieved to date in their other properties too.

For instance, EP-A-540 884 discloses a process for producing multilayercoating systems, especially in the motor vehicle sector usingfree-radically and/or cationically polymerizable, silicone-containingclearcoats, the clearcoat being applied under illumination with lighthaving a wavelength of more than 500 nm or in the absence of light, andthe clearcoat layer subsequently being cured by means of high-energyrays. The surfaces obtained in this way are said to have good opticalproperties and a good scratch resistance.

Similarly, EP-A-568 967 discloses a process for producing multilayercoating systems, especially in the motor vehicle sector, usingradiation-curable clearcoats.

DE 44 21 558, moreover, discloses a coating process in which a primer ispartly crosslinked by means of radiation and then a radiation-curablecoating material is applied. Subsequently, full crosslinking of thetopcoat layer is carried out by means of electron beams. For additionaland final curing of the coating material, furthermore, a concludingoperation of heating the coating layer is recommended.

U.S. Pat. No. 4,675,234, finally, discloses a further multilayer coatingsystem. In this case a basecoat is first of all applied to a substrate,and partly crosslinked by means of radiation, and a topcoat is appliedto this partly crosslinked layer. This topcoat is subjected to radiationcuring along with the basecoat layer.

The clearcoats employed to date do not adhere satisfactorily to thebasecoats. In addition, the resistance of these multilayer systems tocondensation is inadequate. Moreover, any reactive diluents presentpenetrate into the basecoat layer and so partly dissolve it. Theconsequence is an inadequately cured layer system.

SUBJECT OF THE INVENTION

The present invention is therefore based on the object of providing amultilayer coating comprising at least one basecoat layer and onetopcoat layer and being free from the abovementioned disadvantages. Theintention is, in particular, to achieve better adhesion of the baselayer to the substrate and to the topcoat layer.

This object is achieved in that said coating can be produced by

A. applying a radiation-curable component (I) comprising binders basedon polyurethane resins, polyacrylate resins, polyester resins and/oramino resins and also mixtures thereof to a substrate and subjecting itto thermal partial crosslinking or drying at temperatures of less than100° C., preferably 60-80° C., and

B. applying a topcoat to this coating layer, and subsequently carryingout radiation curing.

Basecoat

Preferably, aqueous basecoats are employed. In so far as solvents arepresent, their content is preferably below 15%. Further essentialfeatures are that the basecoat is both radiation-curable and at leastpartially thermally crosslinkable, or drying, at temperatures below 100°C. without irradiation.

Component (I)

Component I comprises binders which are at least partially crosslinkingor drying at temperatures of below 100° C., preferably at temperaturesof 60-80° C. In addition, the binders may be radiation-curable. Thebinders employed in accordance with the invention preferably have adouble-bond density of 0.05-10, more preferably 0.1-4 and, withparticular preference, 0.5-2 mol/kg.

The aromatics content lies preferably below 5%. The amine nitrogencontent is preferably less than 2%, with particular preference below 1%.

It is likewise possible to employ non-radiation-curable binderscomprising radiation-curable components having co-reactive groups, e.g.isocyanates, epoxides, melamines, malonic esters or anhydrides.

Radiation-curable oligomers may also be present in component I. In sofar as the binders are not radiation-curable, such oligomers must beadded.

In the basecoats of the invention the binders are employed preferably inan amount of from 5 to 90% by weight, with particular preference from 20to 70% by weight, based in each case on the overall weight of thetopcoat in the case of clearcoats or on the weight of the coatingcomposition minus pigments and extenders in the case of pigmentedsystems.

As binders, use may be made of water-dilutable or water-dispersiblepolyurethane, polyacrylate, polyester and amino resins, and/or suchresins which can be prepared in organic solution, and also mixturesthereof.

In the text below, the constituents of component I that can be employedin accordance with the invention are described in greater detail:

Polyurethanes

Suitable examples are the polyurethane resins described in theliterature for use in water-based coating materials, especially if thesepolyurethane resins—in modification of the preparation described in therespective literature—can be prepared in the form of organic solutions.

Examples of suitable polyurethane resins are the resins described in thefollowing documents: EP-A-355 433, DE-A 35 45 618, DE-A 38 13 866 andGerman Patent Application DE 4005961.8.

It is preferred to employ water-dilutable polyurethane resins which havea number-average molecular weight (determined by gel permeationchromatography using polystyrene as standard) of from 1000 to 30,000,preferably from 1500 to 20,000, and an acid number of from 5 to 70 mg ofKOH/g, preferably from 10 to 30 mg of KOH/g, and which can be preparedby reaction, preferably chain extension, of isocyanate-functionalprepolymers. Such polyurethane resins are described, for example, inEuropean Patent Application 92904918.7, to which reference is made tothat extent.

Carbamate-functional oligomers are among the preferred polyurethanes.The polyurethanes employed in accordance with the invention preferablyhave a double-bond density of 0.05-10 mol/kg, more preferably 0.1-4mol/kg and, with particular preference, 0.5-2 mol/kg.

Polyacrylates

The polyacrylate resins employed as binders are likewise known and aredescribed, for example, in DE-A 38 32 826. In general, suitablepolyacrylate resins are those which are dilutable or dispersible inwater and can be prepared in the form of organic solutions. They arepreferably polyacrylates comprising radiation-curable components havingco-reactive groups, e.g. isocyanates, epoxides, melamines, malonicesters or anhydrides.

Urethane Acrylates

Urethane(meth)acrylates are well known to the skilled worker and neednot therefore be elucidated further.

The polyurethane acrylates which can be employed in accordance with theinvention can be obtained by reacting a di- or polyisocyanate with achain extender from the group of the diols/polyols and/ordiamines/polyamines and/or dithiols/polythiols and/or alkanolamines andthen reacting the remaining free isocyanate groups with at least onehydroxyalkyl(meth)acrylate or hydroxyalkyl ester of other ethylenicallyunsaturated carboxylic acids.

The amounts of chain extender, di- and/or polyisocyanate andhydroxyalkyl ester are in this case preferably chosen such that

1.) the ratio of equivalents of the NCO groups to the reactive groups ofthe chain extender (hydroxyl, amino and/or mercaptyl groups) liesbetween 3:1 and 1:2, and is preferably 2:1, and

2.) the OH groups of the hydroxyalkyl esters of the ethylenicallyunsaturated carboxylic acids are present in stoichiometric amount inrelation to the remaining free isocyanate groups of the prepolymerformed from isocyanate and chain extender.

It is also possible to prepare the polyurethane acrylates by firstreacting some of the isocyanate groups of a di- or polyisocyanate withat least one hydroxyalkyl ester and then reacting the remainingisocyanate groups with a chain extender. In this case too the amounts ofchain extender, isocyanate and hydroxyalkyl ester are chosen such thatthe ratio of equivalents of the NCO groups to the reactive groups of thechain extender lies between 3:1 and 1:2, and is preferably 2:1, and theratio of equivalents of the remaining NCO groups to the OH groups of thehydroxyalkyl ester is 1:1. All of the forms lying between these twoprocesses are of course also possible. For example, some of theisocyanate groups of a diisocyanate can be reacted first of all with adiol, then a further portion of the isocyanate groups can be reactedwith the hydroxyalkyl ester and, subsequently, the remaining isocyanategroups can be reacted with a diamine.

These various preparation processes for the polyurethane acrylates areknown (cf. e.g. EP-A-204 161) and therefore do not require any moredetailed description. Particular preference is given to aliphaticurethane(meth)acrylates and aliphatic(meth)acrylic esters. Theurethane(meth)acrylates can be flexibilized by, for example, reactingcorresponding isocyanate-functional prepolymers or oligomers withrelatively long-chain, aliphatic diols and/or diamines, especiallyaliphatic diols and/or diamines having at least 6 carbon atoms. Thisflexibilization reaction can be carried out before or after the additionof acrylic and/or methacrylic acid onto the oligomers and/orprepolymers.

Further examples which may be mentioned of suitable binders are thefollowing products which are obtainable commercially:

urethane acrylate Crodamer UVU 300 from Croda Resins Ltd., Kent, GB;aliphatic urethane triacrylate Genomer 4302 from Rahn Chemie, CH;aliphatic urethane diacrylate Ebecryl 284 from UCB, Drogenbos, Belgium;aliphatic urethane triacrylate Ebecryl 294 from UCB, Drogenbos, Belgium;aliphatic urethane triacrylate Roskydal LS 2989 from Bayer AG, Germany;aliphatic urethane diacrylate V94-504 from Bayer AG, Germany; aliphatichexafunctional urethane acrylate Viaktin VTE 6160 from Vianova, Austria;aliphatic urethane diacrylate Laromer 8861 from BASF AG and experimentalmodifications thereof, such as urethane acrylate dispersion Laromer 8949from BASF AG, and Viaktin 6155 from Vianova.

Polyesters

Polyester resins which are dilutable or dispersible in water and can beprepared in the form of organic solutions can also be used as binders.Use is made, for example, of corresponding commercial water-dilutable orwater-dispersible polyester resins and also of the polyester resinswhich are customarily employed in water-based coating materials.

Amino Resins

Water-dilutable or water-dispersible amino resins are also suitable asbinders. It is preferred to employ water-dilutable melamine resins.These are, generally, etherified melamine-formaldehyde condensationproducts. The water solubility of the amino resins depends—apart from onthe degree of condensation, which should be as low as possible—on theetherifying component, with only the lowest members of the alcohol orethylene glycol monoether series giving water-soluble condensates. Mostimportant are the methanol-etherified melamine resins. With the use ofsolubilizers, butanol-etherified melamine resins can also be dispersedin aqueous phase. It is also possible to insert carboxyl groups into thecondensate. Transetherification products of highly etherifiedformaldehyde condensates with oxycarboxylic acids are soluble in waterafter neutralization, by way of their carboxyl groups, and may bepresent in the base paints.

The binders employed may also of course comprise mixtures of theabovementioned binders and, in addition or alone, other water-dilutableor water-dispersible binders.

Radiation-curable Components

The basecoats of the invention may comprise radiation-curable oligomers,such as polyether, polyester and/or polyurethane(meth)acrylates andtheir dispersions. Preference is given to proportions of from 5 to 90%by weight, preferably, in UV-cured formulations, from 10 to 80% byweight, based on the weight of the coating composition minus pigmentsand extenders. Such oligomers must be added when the binders present incomponent I are not radiation-curable.

A binder system for component I that is preferred in accordance with theinvention comprises preferably urethane(meth)acrylates. Particularpreference is given to aliphatic urethane(meth)acrylates andaliphatic(meth)acrylic esters. Preferably, the double-bond density is0.05-10, more preferably 0.1-4 and, with particular preference, 0.5-2mol/kg.

A binder system which is preferred in accordance with the inventioncomprises aliphatic urethane dispersions and non-radiation-curablebinders, preferably polyacrylates, comprising radiation-curablecomponents having co-reactive groups, e.g. isocyanates, epoxides,melamines, malonic esters or anhydrides.

The aromatic content in this case lies preferably below 5%. The aminenitrogen content lies preferably below 2% and, with particularpreference, below 1%.

Customary photoinitiators employed in radiation-curable systems,examples being benzophenones, benzoins or benzoin ethers, preferablyhydroxyacrylic ketones and (bis)acylphosphine oxides, can likewise beused. it is also possible, for example, to employ the productsobtainable commercially under the names Irgacure® 184, Irgacure® 1800and Irgacure® 500 from Ciba Additive, Genocure® MBF from Rahn andLucirin® TPO from BASF AG. When using cationic UV curing it is possibleto employ epoxy systems, such as Cyracure 6110 (Union Carbide), and thecorresponding photoinitiators, e.g. Cyracure 6990.

Further Additional Materials

Specifically, the basecoat may include, in addition, UV absorbers,preferably triazine compounds, and free-radical scavengers. It may alsofeature rheological agents and other coatings auxiliaries. Pigments ofwhatever kind may also of course be incorporated, examples being colourpigments such as azo pigments, phthalocyanine pigments, carbonylpigments, dioxazine pigments, titanium dioxide, pigmentary carbon black,iron oxides and chromium and/or cobalt oxides, or special-effectpigments, such as metal flake pigments, especially aluminium flakepigments, and pearl lustre pigments, and/or liquid-crystalline polymers.The pigment content is 0.5.

The basecoats employed in accordance with the invention may also, ifdesired, include customary auxiliaries, additives, suitable lightstabilizers (e.g. HALS compounds, benzotriazoles, oxalanilides, and thelike), slip additives, polymerization inhibitors, matting agents,defoamers, levelling agents and film-forming auxiliaries, examples beingcellulose derivatives, or other additives that are commonly employed inbasecoats. These customary auxiliaries and/or additives are usuallyemployed in an amount of up to 15% by weight, preferably from 2 to 9% byweight, based on the weight of the coating composition minus pigmentsand minus extenders.

Component (II)

Prior to application it is possible if desired to add a component (II)comprising crosslinking agents, preferably free isocyanates or a mixtureof free polyisocyanates and, with utmost preference, polyisocyanates oflow viscosity. Present as crosslinking agents there may therefore be atleast one unblocked di- and/or polyisocyanate present if desired indispersion or solution in one or more organic solvents which may or maynot be dilutable with water.

The free polyisocyanate constituent comprises any desired organicpolyisocyanates having free isocyanate groups attached to aliphatic,cycloaliphatic, araliphatic and/or aromatic structures. It is preferredto employ polyisocyanates having from 2 to 5 isocyanate groups permolecule and viscosities of from 100 to 2000 mPa.s (at 23 degrees C.).If desired, small amounts of organic solvent, preferably from 1 to 25%by weight based on pure polyisocyanate, can be added to thepolyisocyanates in order to improve the ease of incorporation of theisocyanate and, if desired, to lower the viscosity of the polyisocyanateto a level which is within the abovementioned ranges. Examples ofsolvents suitable as additives for the polyisocyanates are ethoxyethylpropionate, butyl acetate and the like. Examples of suitable isocyanatesare described, for example, in “Methoden der organischen Chemie”[Methods of Organic Chemistry], Houben-Weyl, Volume 14/2, 4th edition,Georg Thieme Verlag, Stuttgart 1963, page 61 to 70, and by W. Siefken,Liebigs Ann. Chem. 562, 75 to 136. Suitable, for example, are theisocyanates referred to in the description of the polyurethane resins(A2) and/or isocyanate-functional polyurethane prepolymers which can beprepared by reacting polyols with an excess of polyisocyanates and whichare preferably of low viscosity. It is also possible to employpolyisocyanates which have isocyanurate and/or biuret and/or allophanateand/or urethane and/or urea and/or uretdione groups. Polyisocyanateshaving urethane groups, for example, are obtained by reacting some ofthe isocyanate groups with polyols, such as trimethylolpropane andglycerol, for example. It is preferred to employ aliphatic orcycloaliphatic polyisocyanates, especially hexamethylene diisocyanate,dimerized and trimerized hexamethylene diisocyanate, isophoronediisocyanate, 2-isocyanatopropylcyclohexyl isocyanate,dicyclohexylmethane 2,4′-diisocyanate or dicyclohexylmethane4,4′-diisocyanate or mixtures of these polyisocyanates. Very particularpreference is given to the use of mixtures of uretdione- and/orisocyanurate- and/or allophanate-functional polyisocyanates based onhexamethylene diisocyanate, as are formed by catalytic oligomerizationof hexamethylene diisocyanate using appropriate catalysts. Thepolyisocyanate constituent may also consist, furthermore, of any desiredmixtures of the free polyisocyanates given by way of example.

Crosslinking agents that are suitable in accordance with the invention,other than the abovementioned crosslinking agents, includetris(alkoxycarbonylamino)triazines of the formula

where R=methyl, butyl- . . . groups. It is also possible to employderivatives of the said compounds. It is preferred to employtris(alkoxycarbonylamino)triazines as are described in U.S. Pat. No.5,084,541.

The carbamate groups react preferentially with OH carriers and, inparticular, with hydroxyl groups having as little steric hindrance aspossible. Amino groups cannot be crosslinked by thetris(alkoxycarbonylamino)triazine. Instead, there is elimination of thecarbalkoxy group. For drying or partial crosslinking at temperatures ofless than 100° C. it is preferable to add suitable catalysts.

Alternatively, in addition to the isocyanates described above, blockedisocyanate or a mixture of blocked polyisocyanates may be present incomponent (II). Compounds suitable for this purpose include thosedescribed above in connection with component (I).

A preferred embodiment of the invention is characterized in that theamounts of the crosslinking agents are 5-30 parts, preferably 20 parts,based on 100 parts of binder, solids/solids. In addition to thecrosslinking agents mentioned, blocked crosslinking systems may also bepresent in component (I) and/or (II).

If blocked isocyanate is admixed, it is preferably designed so that itincludes both isocyanate groups blocked with a blocking agent (Z1) andthose blocked with a blocking agent (Z2), the blocking agent (Z1) beinga dialkyl malonate or a mixture of dialkyl malonates, the blocking agent(Z2) being a blocking agent which is different from (Z1) and containsactive methylene groups, or an oxime, or a mixture of these blockingagents, and the ratio of equivalents between the isocyanate groupsblocked with (Z1) and the isocyanate groups blocked with (Z2) liesbetween 1.0:1.0 and 9.0:1.0, preferably between 8.0:2.0 and 6.0:4.0 and,with particular preference, between 7.5:2.5 and 6.5:3.5:. For drying orpartial crosslinking at temperatures of 100° C., these blockedisocyanates are employed with the addition of appropriate catalysts.

The blocked isocyanate is prepared preferably as follows. Apolyisocyanate or a mixture of polyisocyanates is reacted conventionallywith a mixture of the blocking agents (Z1) and (Z2), the mixture of theblocking agents (Z1) and (Z2) comprising the blocking agents (Z1) and(Z2) in a molar ratio which lies between 1.0:1.0 and 9.0:1.0, preferablybetween 8.0:2.0 and 6.0:4.0 and, with particular preference, between7.5:2.5 and 6.5:3.5. The polyisocyanate or the mixture ofpolyisocyanates can be reacted with the mixture of the blocking agents(Z1) and (Z2) to an extent such that isocyanate groups can no longer bedetected. In practice, this may require the use of very large excessesof blocking agents and/or very long reaction times. It has been foundthat coating materials having good properties are obtained even when atleast 50, preferably at least 70 percent of the isocyanate groups of thepolyisocyanate or of the mixture of polyisocyanates are reacted with themixture of the blocking agents (Z1) and (Z2) and the remainingisocyanate groups are reacted with a hydroxyl-containing compound or amixture of hydroxyl-containing compounds. Hydroxyl-containing compoundsemployed are preferably low molecular mass aliphatic or cycloaliphaticpolyols, such as neopentyl glycol, dimethylolcyclohexane, ethyleneglycol, diethylene glycol, propylene glycol,2-methyl-2-propylpropane-1,3-diol, 2-ethyl-2-butylpropane-1,3-diol,2,2,4-trimethylpentane-1,5-diol and 2,2,5-trimethylhexane-1,6-diol, orthe hydroxyl-containing binder which can be employed as constituent (1).A suitable blocked polyisocyanate is also obtainable by mixingpolyisocyanates blocked with the blocking agent (Z1) and (Z2) in a ratiosuch that the mixture obtained features a ratio of equivalents betweenthe isocyanate groups blocked with (Z1) and the isocyanate groupsblocked with (Z2) of between 1.0:1.0 and 9.0:1.0, preferably between8.0:2.0 and 6.0:4.0 and, with particular preference, between 7.5:2.5 and6.5:3.5. In principle, all polyisocyanates which can be employed in thepaints field can be employed for preparing the blocked polyisocyanate.It is preferred, however, to employ polyisocyanates whose isocyanategroups are attached to aliphatic or cycloaliphatic radicals. Examples ofsuch polyisocyanates are hexamethylene diisocyanate, isophoronediisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethanediisocyanate and 1,3-bis(2-isocyanatoprop-2-yl)benzene (TMXDI) and alsoadducts of these polyisocyanates with polyols, especially low molecularmass polyols, such as trimethylolpropane, for example, and isocyanurate-and/or biuret-functional polyisocyanates which can be derived from thesepolyisocyanates. Polyisocyanates employed with particular preference arehexamethylene diisocyanate and isophorone diisocyanate, isocyanurate- orbiuret-functional polyisocyanates derived from these diisocyanates andcontaining preferably more than two isocyanate groups in the molecule,and also reaction products of hexamethylene diisocyanate and isophoronediisocyanate or of a mixture of hexamethylene diisocyanate andisophorone diisocyanate with 0.3-0.5 equivalents of a low molecular masspolyol having a molecular weight of from 62 to 500, preferably from 104to 204, in particular a triol, such as trimethylolpropane, for example.Dialkyl malonates or a mixture of dialkyl malonates are employed asblocking agent (Z1). Examples of dialkyl malonates which can be employedare dialkyl malonates having in each case 1 to 6 carbon atoms in thealkyl radicals, such as dimethyl malonate and diethyl malonate, forexample, preference being given to the employment of diethyl malonate.As blocking agents (Z2), blocking agents which are different from (Z1)and contain active methylene groups, and oximes, and also mixtures ofthese blocking agents, are employed. As examples of blocking agentswhich can be employed as blocking agents (Z2) mention is made of:methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decylor dodecyl acetoacetate, acetone oxime, methyl ethyl ketoxime,acetylacetone, formaldoxime, acetaldoxime, benzophenoxime, acetoxime anddiisobutyl ketoxime. As blocking agent (Z2) it is preferred to employ analkyl acetoacetate having 1 to 6 carbon atoms in the alkyl radical, or amixture of such alkyl acetoacetates, or a ketoxime or mixture ofketoximes. Particular preference is given to ethyl acetoacetate ormethyl ethyl ketoxime for use as blocking agent (Z2).

Preparation

The two components (I) and (II) are prepared by the customary methodsfrom the individual constituents with stirring. The preparation of thebasecoat from the individual components (I) and (II) takes placelikewise by means of stirring or dispersion using commonly employedapparatus, for example by means of dissolvers or the like, or by meansof the likewise customarily employed 2-component metering and mixingunits, or by means of the process for preparing aqueous 2-componentpolyurethane coating materials that is described in DE-A-195 10 651,page 2, line 62, to page 4, line 5.

The components, especially component (I), can be formulated asnonaqueous components (i.e. with organic solvents) or as aqueouscomponents. In the case of a nonaqueous formulation it is possible touse the organic solvents that are customary in paint preparation. Usingaqueous components, aqueous coating compositions are obtained aftercomponents (I) and (II) have been mixed. If an aqueous coatingcomposition is desired, the components (I) and/or (II) may alternativelybe formulated in substantially water-free form and substantially freefrom organic solvents, and yet in water-dispersible form. In that casethe aqueous coating composition is obtained by mixing the components andadding water. A component (I) soluble or dispersible in water can beformulated in a conventional manner by introducing, for example, acidicgroups into the binder which are then neutralized with a customary base,such as ammonia or an organic amine such as triethylamine. Thedissolving or dispersing of a water-dispersible component (I) and/or(II) in water takes place in a conventional manner by means, forexample, of vigorous stirring with or without gentle heating.Alternatively, dissolving or dispersing may take place in water by meansof nonionic emulsifiers. To this extent, reference is again made tostandard techniques for preparing aqueous coating materials.

Use

The basecoats of the invention are suitable for the direct coating of asubstrate. In this case, it desired, component (I) and (II) are mixedwith one another to form the coating composition prior to application,preferably directly before application, this coating composition isapplied, and the applied composition is dried for 5-30 minutes,preferably 10 minutes, at less than 100° C., preferably 60-80° C.Component (I) and component (II) can if desired be applied by means of aspecial two-component applicator, then dried and cured together.

The aqueous coating materials prepared using the component system of theinvention can be applied to glass and to a very wide variety of metalsubstrates, such as aluminium, steel, various alloys of iron, and thelike. It is likewise possible to coat primed or unprimed plastics suchas ABS, AMMA, ASA, CA, CAB, EP, UF, CF, MF, MPF, PF, PAN, PA, PC, PE,HDPE, LDPE, LLDPE, UHMWPE, PET, PMMA, PP, PS, SB, PUR, PVC, RF, SAN,PBT, PPE, POM, PUR-RIM, SMC, BMC, PP-EPDM and UP (abbreviationsaccording to DIN 7728T1), for example.

The plastics to be coated can of course also be polymer blends, modifiedplastics or fibre-reinforced plastics. Preferably, the coatingcompositions of the invention are used to coat polycarbonate blends andpolypropylene blends, for example. The coating compositions of theinvention are employed in particular for the plastics that are commonlyemployed in vehicle construction, especially motor vehicle construction.

In the case of unfunctionalized and/or apolar substrate surfaces, thesesubstrates should be subjected to pretreatment, such as by plasma orflaming, prior to coating.

Suitable primers in this context are all customary primers, includingboth conventional and aqueous primers. It is of course also possible toemploy radiation-curable conventional and radiation-curable aqueousprimers.

Finally, the coating compositions may be applied to other substrates aswell, such as metal, wood or paper, for example. Application takes placewith the aid of customary methods, for example by spraying, casting,knifecoating, dipping or brushing.

According to the specified uses of the component system, according tothe invention, the invention also embraces binders and coating materialsprepared with this system and also articles coated with these bindersand coating materials.

The coating compositions of the invention are dried normally attemperatures of less than 100° C. preferably at temperatures of 60-80°C. Curing of the basecoat is carried out subsequently with radiation,preferably UV light or electron beams. If desired, radiation curing maybe carried out only after a topcoat layer has been applied.

The basecoat described is notable for surprisingly improved propertiesand, in particular, by better adhesion of the coating layer to thecoated substrates. Similarly, the adhesion of coating layers applied tothe basecoat layer is better. In so far as reactive diluents are presentin the next coating layer, they are surprisingly unable to startdissolving the basecoat. In addition, when coated with a topcoat, thebasecoat exhibits enhanced condensation resistance. The swelling of thebase layer is also reduced. In addition, the brightness of colour andthe colour effects are improved.

Topcoat

In accordance with the invention, a radiation-curable topcoat layer isapplied to the basecoat layer. This topcoat layer is preferably aclearcoat. A composition which can be employed with preference for thistopcoat layer is elucidated further below.

The solids content of the topcoat is . . . , . . . It is preferred toemploy water-dilutable binders. In so far as solvents are present, theircontent is below 15%.

It is selected such that the cured topcoat has a storage modulus E′ inthe elastomeric range of at least 10^(7.6) Pa, preferably of at least10^(8.0) Pa and, with particular preference, of at least 10^(8.3) Pa,and a loss factor at 20° C. of not more than 0.10, preferably not morethan 0.06, the storage modulus E′ and the loss factor tan δ having beenmeasured by dynamomechanical thermoanalysis on homogeneous free filmshaving a thickness of 40±10 μm. The loss factor tan δ is defined here asthe quotient of the loss modulus E″ and the storage modulus E′.

Dynamomechanical thermoanalysis is a widely known method of determiningthe viscoelastic properties of coatings and is described, for example,in Murayama, T., Dynamic Mechanical Analysis of Polymeric Material,Elsevier, New York, 1978 and Loren W. Hill, Journal of CoatingsTechnology, Vol. 64, No. 808, May 1992, pages 31 to 33.

The measurements can be made using, for example, the instruments MK II,MK III or MK IV from Rheometrics Scientific.

The radiation-curable topcoat described, having the correspondingviscoelastic properties referred to above, are preferably curable bymeans of UV or electronic radiation, especially by means of UVradiation. In addition, topcoats based on ormocers, inter alia, are alsosuitable, for example.

These radiation-curable topcoats normally include at least one andpreferably two or more radiation-curable binders based in particular onethylenically unsaturated prepolymers and/or ethylenically unsaturatedoligomers, alone or together with one or more reactive diluents, with orwithout one or more photoinitiators and with or without customaryauxiliaries and additives.

It is preferred to employ radiation-curable coating compositions whoseviscosity at 23° C. is less than 100 s flow time in the DIN 4 cup, withparticular preference less than 80 s flow time in the DIN 4 cup.

Binders

Examples of binders employed in these radiation-curable coatingcompositions are (meth)acrylofunctional (meth)acrylic copolymers,polyether acrylates, polyester acrylates, unsaturated polyesters, epoxyacrylates, urethane acrylates, amino acrylates, melamine acrylates,silicone acrylates and the corresponding methacrylates. It is preferredto employ binders which are free from aromatic structural units. The useof epoxy acrylates leads to coatings which, although hard and scratchresistant, generally exhibit a level of weathering stability that is inneed of improvement. Preference, therefore, is given to employingurethane(meth)acrylates and/or polyester(meth)acrylates, the use ofaliphatic urethane acrylates being particularly preferred. Preference isalso given to the use of substantially silicone-free and, withparticular preference, silicone-free binders, since the resultingtopcoats then possess an overcoatability which is improved relative tothat of silicone-containing topcoats, especially on refinish.

The polymers or oligomers employed as binders normally have anumber-average molecular weight of from 500 to 50,000, preferably from1000 to 5000.

The polymers and/or oligomers employed in the topcoats of the inventionpreferably have at least 2 and, with particular preference, from 3 to 6double bonds per molecule. The binders used preferably also have adouble bond equivalent weight from 400 to 2000, with particularpreference from 500 to 900. In addition, the binders have a viscosity at23° C. which is preferably from 250 to 11,000 mPa.s.

Polyester Acrylates

Polyester(meth)acrylates are known in principle to the skilled worker.They can be prepared by various methods. For example, acrylic acidand/or methacrylic acid can be employed directly as acid component whensynthesizing the polyesters. In addition there exists the possibility ofemploying hydroxyalkyl esters of (meth)acrylic acid as alcohol componentdirectly when synthesizing the polyesters. Preferably, however, thepolyester(meth)acrylates are prepared by acrylicization of polyesters.For example, it is possible first of all to synthesizehydroxyl-containing polyesters, which are then reacted with acrylic ormethacrylic acid. It is also possible first of all to synthesizecarboxyl-containing polyesters, which are then reacted with ahydroxyalkyl ester of acrylic or methacrylic acid. Unreacted(meth)acrylic acid can be removed from the reaction mixture by washing,distillation or, preferably, by reaction with an equivalent amount of amono- or diepoxide compound using appropriate catalysts, such astriphenylphosphine, for example. For further details of the preparationof polyester acrylates reference may be made in particular to DE-A 33 16593 and DE-A 38 36 370 and also to EP-A-54 105, DE-B 20 03 579 andEP-B-2866.

Polyether Acrylates

Polyether(meth)acrylates are likewise known in principle to the skilledworker. They can be prepared by various methods. For example,hydroxyl-containing polyethers which are esterified by acrylic acidand/or methacrylic acid can be obtained by reacting dihydric and/orpolyhydric alcohols with various amounts of ethylene oxide and/orpropylene oxide by well-known methods (cf. e.g. Houben-Weyl, Volume XIV,2, Makromolekulare Stoffe [Macromolecular substances] II, (1963)). It isalso possible to employ products of the addition polymerization oftetrahydrofuran or of butylene oxide.

The polyether(meth)acrylates and the polyester(meth)acrylates can beflexibilized, for example, by reacting corresponding OH-functionalprepolymers or oligomers (based on polyether or polyester) withrelatively long-chain aliphatic dicarboxylic acids, especially aliphaticdicarboxylic acids having at least 6 carbon atoms, examples beingadipic, sebacic, dodecanedioic and/or dimeric fatty acids. Thisflexibilization reaction can be carried out before or after the additionof acrylic and/or methacrylic acid onto the oligomers and/orprepolymers.

Epoxy Acrylates

Epoxy(meth)acrylates are also well known to the skilled worker andtherefore require no further elucidation. They are normally prepared byaddition reaction of acrylic acid with epoxy resins, examples beingepoxy resins based on bisphenol A, or other commercially customary epoxyresins.

The epoxy(meth)acrylates can be flexibilized analogously by, forexample, reacting corresponding epoxy-functional prepolymers and/oroligomers with relatively long-chain, aliphatic dicarboxylic acids,especially aliphatic dicarboxylic acids having at least 6 carbon atoms,examples being adipic, sebacic, dodecanedioic and/or dimeric fattyacids. This flexibilization reaction can be carried out before or afterthe addition of acrylic and/or methacrylic acid onto the oligomersand/or prepolymers.

It is possible, furthermore, to employ the urethane acrylates, describedabove and used in the basecoats, for topcoats as well.

Reactive Diluents

The topcoats of the invention may, it desired, include one or morereactive diluents. These reactive diluents can be ethylenicallyunsaturated compounds. The reactive dilients can be mono-, di- orpolyunsaturated. They serve customarily to influence the viscosity andthe technical properties of the coating material, such as thecrosslinking density, for example.

The reactive diluent or diluents are employed in the topcoats of theinvention preferably in an amount of from 0 to 70% by weight and, withparticular preference, from 15 to 65% by weight, based in each case onthe overall weight of the topcoat in the case of the clearcoats or onthe weight of the topcoat minus pigments and extenders in the case ofpigmented systems.

Examples of reactive diluents employed are (meth)acrylic acid and estersthereof, maleic acid and its esters and/or monoesters, vinyl acetate,vinyl ethers, vinylureas and the like. Examples are alkylene glycoldi(meth)acrylate, polyethylene glycol di(meth)acrylate, 1,3-butanedioldi(meth)acrylate, vinyl(meth)acrylate, allyl(meth)acrylate, glyceroltri(meth)acrylate, trimethylolpropane tri(meth)acrylate,trimethylolpropane di(meth)acrylate, styrene, vinyltoluene,divinylbenzene, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipropylene glycol di(meth)acrylate, tripropyleneglycol di(meth)acrylate, hexanediol di(meth)acrylate, ethoxyethoxyethylacrylate, N-vinylpyrrolidone, phenoxyethyl acrylate, dimethylaminoethylacrylate, hydroxyethyl(meth)acrylate, butoxyethyl acrylate,isobornyl(meth)acrylate, dimethylacrylamide and dicyclopentyl acrylate,and the long-chain linear diacrylates described in EP-A-250 631, havinga molecular weight of 400 to 4000, preferably 600 to 2500. The twoacrylate groups can, for example, be separated by a polyoxybutylenestructure. It is also possible to employ decyl 1,10-diacrylate anddodecyl 1,12-diacrylate, and the reaction product of 2 moles of acrylicacid with one mole of a dimeric fatty alcohol having generally 36 carbonatoms. Mixtures of the abovementioned monomers are also suitable.

Preferred reactive diluents are mono- and/or diacrylates, such asisobornyl acrylate, hexanediol diacrylate, tripropylene glycoldiacrylate, Laromer® 8887 from BASF AG and Actilane® 423 from AkcrosChemicals Ltd., GB. It is particularly preferred to employ isobornylacrylate, hexanediol diacrylate and tripropylene glycol diacrylate.

Radiation-curable Components

The topcoats of the invention comprise—preferably in proportions of from0 to 10% by weight, in UV-cured formulations preferably from 2 to 60% byweight, based on the weight of the coating composition minus pigmentsand extenders—customary photoinitiators employed in radiation-curablecoating compositions, examples being benzophenones, benzoins or benzoinethers, preferably hydroxyacrylic ketones and bis(acyl)phosphine oxidesin UV formulations. It is also possible, for example, to employ theproducts obtainable commercially under the names Irgacure® 184,Irgacure® 1800 and Irgacure® 500 from Ciba Geigy, Genocure® MBF fromRahn and Lucirin® TPO from BASF AG.

Further, Additional Substances

The topcoats of the invention may if desired also include customaryauxiliaries and/or additives, examples being light stabilizers (e.g.HALS compounds, benzotriazoles, oxalanilide, and the like), slipadditives, polymerization inhibitors, matting agents, defoamers,levelling agents and film-forming auxiliaries, examples being cellulosederivatives, or other additives that are commonly employed in topcoats.These customary additives and/or auxiliaries are usually employed in anamount of up to 15% by weight, preferably from 2 to 9% by weight, basedon the weight of the topcoat minus pigments and minus extenders.

Use

The topcoats of the invention are employed in particular as clearcoats,so that they normally contain only transparent extenders, if any at all,and contain no hiding pigments. Use in the form of pigmented topcoats,however, is also possible. In this case the topcoats contain from 2 to40% by weight, based on the overall weight of the topcoat, of one ormore pigments. Also, in this case the topcoats may include from 1 to 20%by weight, based on the overall weight of the topcoat, of one or moreextenders.

In order to prepare coatings, the topcoats of the invention are appliedto substrates coated with basecoat according to the invention, examplesof such substrates being metal panels or metal strips and plastics ofwhatever kind.

The layer thicknesses of the basecoat are, in accordance with theinvention, between 5 and 100 μm, preferably from 10 to 50 μm and, withparticular preference, from 15 to 30 μm.

The present invention therefore also provides a process for producingmultilayer coating systems, in which process

A. a component (I) comprising binders, which may or may not beradiation-curable and which are based on polyurethane resins,polyacrylate resins and amino resins and also mixtures thereof andpreferably, if the binder is not radiation-curable, a radiation-curablecomponent, preferably a radiation-curable oligomer, is mixed if desiredwith a component (II) comprising free polyisocyanate, the mixture isapplied to a substrate and the applied layer is subjected if desired toradiation curing at temperatures of less than 100° C., preferably 60-80°C., and

B. to this coating layer there is applied a topcoat, which is subjectedto radiation curing.

The coating films are cured by means of radiation, preferably by meansof UV radiation. The apparatus and conditions for these curing methodsare known from the literature (cf. e.g. R. Holmes, U.V. and E.B. CuringFormulations for Printing Inks, Coatings and Paints, SITA Technology,Academic Press, London, United Kingdom 1984) and require no furtherdescription.

In so far as radiation curing of the basecoat layer has not been carriedout prior to applying the topcoat layer, it can then take place in onestep together with the curing of the topcoat layer.

The multilayer coatings of the invention are preferably employed ascoatings in the field of automotive finishing (automotive OEM finishingand refinishing). In addition to their application to a very widevariety of metals, the coating compositions can of course also beapplied to other substrates, such as wood, paper, plastics, mineralsubstrates or the like, for example. They are, furthermore, susceptibleof employment in the field of the coating of packaging containers aswell, and in the field of the coating of films for the furnitureindustry, vehicle components and the like.

The multilayer coating systems of the invention are particularlysuitable for use in the field of the motor-vehicle OEM finishing and/ormotor-vehicle refinishing of car bodies and parts thereof and also lorrybodies and the like.

The invention is elucidated further below with reference to exemplaryembodiments. In these embodiments, all parts are by weight unlessexpressly stated otherwise.

Red Variations for UV Clearcoat Applications

a) Imperial Red—Standard (obtainable from BASF Coatings AG in Münster,polyurethane dispersion which crosslinks on temperature exposure withmelamine condensate resins)

b) Imperial Red—Standard+7.5% isocyanate based on overall weight(Basonat HI 100, obtainable from BASF Coatings AG in Münster, 100%solids content, NCO content 22.5% HDI trimer)

a)+b) dried initially at 80° C. for 10 minutes

c) as a), but polyurethane resin substituted 1:1 for 30% by weight(based on overall coating material) radiation-curable urethane acrylatedispersion (Laromer 8949, obtainable from BASF AG, polyurethanedispersion with free acrylate groups) , UV initiator (Irgacure®184+Lucirin LR 8893)+7.5% isocyanate based on overall weight (Basonat HI100, obtainable from BASF AG, 100% solids content NCO content 22.5% HDItrimer)

d) as c), but without isocyanate

c)+d) dried initially at 80° C. for 10 minutes+UV curing at 1500 mJ/cm²

UV and clearcoat is employed in all examples:

=>Laromer 8861 (BASF AG, urethane diacrylate 80% in hexanedioldiacrylate) reactive diluent, light stabilizer additives (Tinuvin 400,Tinuvin 292 from Ciba), photoinitiator (Irgacure® 164 from Ciba)

=>UV curing

Result

a) before clearcoat application: soft, not scratch-proof after clearcoatapplication: visually satisfactory, but very soft, crosshatchsatisfactory after constant alternating climate test (40° C., 100%atmospheric humidity): mate cracking, crosshatch unsatisfactory

b) before clearcoat application: scratchproof only after 2 days afterclearcoat application: slight texture, fully cured, crosshatchsatisfactory after alternating climate test: texture, softening,crosshatch unsatisfactory

c) before clearcoat application: scratchproof after clearcoatapplication: visually satisfactory, crosshatch satisfactory afteralternating climate test: crosshatch satisfactory

d) before clearcoat application: soft after clearcoat application:visually satisfactory, very soft after alternating climate test: fullsubfilm creep with H₂O

SUMMARY

The batches with isocyanate exhibit a markedly better resistance tocondensation.

Formulation: Paste mixture 21.0 Butyl glycol 13.0 Levelling agent 1.0Deionized water 4.0 Polyurethane acrylate 57.5 Irgacure 184 3.0Photoinitiator 5.0 Deionized water 10.0 (polyisocyanate crosslinker)

What is claimed is:
 1. Multilayer coating comprising at least onebasecoat layer and one topcoat layer prepared by A. applying a basecoatto a substrate and subjecting the basecoat to thermal partialcrosslinking or drying at temperatures of less than 100° C., and B.applying a topcoat to the basecoat, and subsequently carrying outradiation curing; wherein the basecoat comprises a radiation-curablecomponent (I) comprising a first binder that is at least one of: i) aradiation-curable resin that is at least one of a polyurethane resin, apolyacrylate resin, a polyester resin, and an amino resin, and ii) aradiation-curable oligomer, and a second binder that is different fromthe first binder comprising a radiation-curable component havingco-reactive groups thereon that are at least one of an isocyanate, amelamine, a malonic ester, and an anhydride.
 2. A multilayer coalingaccording to claim 1, wherein the first binder comprises theradiation-curable oligomer.
 3. A multilayer coating according to claim1, wherein the first binder comprises the radiation-curable resin.
 4. Amultilayer coating according to claim 1, wherein the double-bond densityof at least one of the first binder and the second binder is 0.05-10mol/kg.
 5. A multilayer coating according to claim 1, wherein the secondbinder is a polyacrylate that has the radiation curable co-reactivegroups thereon.
 6. A multilayer coating according to claim 1, whereinthe co-reactive groups further comprise an epoxide.
 7. A multilayercoating according to claim 1, wherein the basecoat further comprisesfree polyisocyanate.
 8. A multilayer coating according to claim 1,wherein the basecoat comprises water-dilutable binders.
 9. A multilayercoating according to claim 1, wherein the solvent content lies below3.5%.
 10. A multilayer coating according to claim 1, wherein thebasecoat and the topcoat are curable by means of UV radiation orelectron beam radiation.
 11. A multilayer coating according to claim 1,wherein the double-bond density of at least one of the first binder andthe second binder is 0.1-4 mol/kg.
 12. A multilayer coating according toclaim 1, wherein the basecoat further comprises component II, acrosslinking agent.
 13. A multilayer coating according to claim 12wherein the crosslinking agent is an unblocked di- or polyisocyanate.14. A multilayer coating according to claim 12 further comprisingblocked di- or polyisocyante compounds.
 15. A process for producing acoated substrate with the multilayer coating of claim 1 comprising (a)applying to a substrate the basecoat composition, (b) subjecting saidbasecoat to thermal partial crosslinking or drying at temperatures offrom 60-80° C., (c) subsequently applying to the basecoat, a topcoat and(d) subsequently curing the coating layers with radiation.
 16. A coatedsubstrate, comprising a motor vehicle or motor vehicle component havingthereon a multilayer coating of claim 2.